The resurgence of Rafael Nadal and Roger Federer this season is all about turning a liability into an asset.

If tennis were investing, these two would be the master traders of 2017, the best on tour at taking a losing stock and turning it into a winning one.

The stock in focus is second serves.

Second serves are generally underrated, underperforming and under the radar when it comes to the discussion of tennis greats. Second serves for most of the tennis market are a speculative stock, but not for Nadal and Federer.

Nadal, who returned to the No. 1 ranking this week, and Federer, who is ranked third, have hoarded the Grand Slam singles titles this year and are among the favourites at the U.S. Open, which begins Monday.

There is only one serve, return or “under pressure” statistic on the ATP Tour in which Nadal and Federer are ranked one-two this season: second-serve points won.

Nadal has won an impressive 61 per cent of his second-serve points this season to lead the tour. Federer is right behind, having won 60 per cent. Each player’s career average is 57 per cent, and Nadal in particular has improved from last season, when he won 54 per cent of his second-serve points.

They double-down on making a derivative of their game a peak performer. The rest of the tour tend to struggle mightily.

Take the 2016 U.S. Open as an example.

Performance behind first serves in New York in the men’s draw last year was outstanding, as always. There were 15,749 first serves hit in the court, with the server winning 11,209 of them, accounting for a dominant win percentage of 71 per cent.

First serves are a cash cow, a stalwart of the portfolio.

At the 2016 Open, second serves were a losing prospect. There were 12,256 second serves hit last year, with the server winning 6,049 of the points, or 49.3 per cent. Returners won more points than servers did when the point started with a second serve.

Winning points behind first serves is a given. Winning points behind a second serve is a privilege.

At the 2017 Australian Open, Federer won the tournament on the strength of winning 55 per cent of his second-serve points. Nadal won the next Grand Slam event, the French Open, winning a ridiculously high 74 per cent of his second-serve points. Federer then won Wimbledon, winning 67 per cent of his second-serve points.

Talk about a return on investment.

Second-serve performance is crucial to winning Grand Slam tournaments because second serves are the gateway for the returner to win crucial return points to break serve.

Nadal and Federer direct their second serves primarily at their opponents’ body on the backhand side, trying to jam them up while staying away from the danger of the centreline and sideline, and also staying clear of the opponent’s more potent forehand return.

Federer’s right-handed second serve is hit with topspin, kicking up high into the body on the backhand side of the returner. Nadal’s left-handed second serve is all slice, or side spin, either meant to move away from the returner, or chase him as he tries to move away from it.

For Nadal or Federer to win again in New York this year, performance behind their second serves will matter more than their scorching forehand winners, aces or adventures to the net.

Second serves are where the smart money lives. Over time, the best gains are accrued by limiting losses, and that is what separates Federer and Nadal from the field this year.